Automobile part made from titanium

ABSTRACT

A titanium auto part having a strength equal to the conventional JIS Type 2 and Type 3 and a phase structure enabling design of similar parts and processes and having a high residual ductility after being worked. A titanium auto part characterized by containing, by wt %, Fe: 0.15 to 0.5%, nitrogen: 0.015 to 0.04%, oxygen, and the balance of titanium and unavoidable impurities, wherein, when the Fe content is [Fe], the nitrogen content is [N], and the oxygen content is [O], the oxygen equivalent value Q=[O]+2.77 [N]+0.1 [Fe] is Q: 0.11 to 0.28%. Q: 0.11 to 0.20% is preferable. As the auto part, the invention is suitably applied to any of a muffler, exhaust pipe, motorcycle frame, motorcycle rear fork, and motorcycle front fork inner pipe.

TECHNICAL FIELD

[0001] The present invention relates to an auto part comprised oftitanium characterized by a light weight and high strength. Inparticular, it relates to an auto part comprised of a JIS Type 2 or Type3 commercially pure titanium or a low alloy titanium comprised of one ofthese plus a small amount of Fe.

BACKGROUND ART

[0002] Titanium materials may be classified into alloy titanium andcommercially pure titanium. Among these, commercially pure titanium hasthe characteristics of being comparatively superior in workability aswell as having a moderate degree of strength characteristics andsuperior corrosion resistance and being relatively easy to weld as well.It has been worked into various shapes such as medium thick plates, hotrolled and cold rolled strips, thin sheets cut from the same, weldedpipes obtained by shaping and welding these thin sheets, large diameterround bars, square bars, bar and wire coils, medium and small diameterbars and wires cut from the same, and seamless pipes made by hotextrusion for use in the fields making use of their light weight, highstrength, high corrosion resistance, and other characteristics such asaircraft, chemicals, marine structures, electric power, etc. where suchcharacteristics are required. Further, recently, remarkable progress hasbeen made in application to automobiles and other consumer products.Various titanium parts are now being made frequent use of in thesefields.

[0003] Such commercially pure titanium is classified into JIS Type 1 toType 4 according to the added elements and strength characteristics. Inthe most general use JIS Type 2, the material is limited to a maximum of0.20 wt % of oxygen, a maximum of 0.05 wt % of nitrogen, and a maximumof 0.25 wt % of Fe. Further, JIS Type 3 having a higher strength thanJIS Type 2 is limited to a maximum of 0.30 wt % of oxygen, a maximum of0.07 wt % of nitrogen, and a maximum of 0.30 wt % of Fe. (Note that inthe following description, the amounts of chemical components are wt %.)

[0004] In actuality, however, these JIS Type 2 and Type 3 commerciallypure titaniums only contain at most 0.015% of nitrogen and at most 0.1%of Fe at the present time. With the exception of containing 0.07 to 0.3%of oxygen and unavoidable impurities, they are literally pure titanium.

[0005] These conventional JIS Type 2 and JIS Type 3 commercially puretitaniums, as explained above, are formed into products of varioussectional shapes and dimensions and are being used in a broad spectrumof fields when applying them to mufflers, exhaust pipes, motorcycle rearforks, frames, and other auto parts etc., they are worked intocomplicated shapes by bending, forging, flat rolling (cold rolling barand wire of circular cross-sections into sheet shapes), pipecontraction, drawing, pipe expansion, punching, bore-expanding, andother cold working processes. Therefore, the ductility is consumed inthe working and sometimes the ductility of the part after being workedbecomes small partially. On the other hand, auto parts will probably beused under increasingly severe environments in the future, so parts witha large residual ductility have been strongly sought.

[0006] To make the residual ductility of a part after being workedincrease, it is necessary to improve the original ductility of thematerial. Regarding the latter, in high strength titanium alloys, as amethod for securing a high ductility, as described in Japanese NationalPublication (Tokuhyo) No. 8-833292 (International Publication No.W096/33292), there is the method of simultaneously adding Fe andnitrogen. This method is believed to be able to be applied even totitanium materials of a strength level corresponding to the loweststrength JIS Type 2 or Type 3, but if adding close to 1% of Fe as inthis method, a large amount of the β-phase (bcc crystal structure) of adifferent crystal structure from the α-phase (hcp crystal structure) ofthe matrix is produced, the modulus of elasticity and heat expansioncharacteristics become different from the conventional case ofcontaining only the α-phase as the main phase, and the problem arisesthat it is necessary to readjust the shape of the part or productionprocess or method of working so as to secure compatibility with thesurrounding parts.

DISCLOSURE OF INVENTION

[0007] The present invention provides a titanium auto part comprised ofa titanium material having a strength equal to the conventional JIS Type2 and Type 3 and a phase structure enabling design of similar parts andprocesses as conventional JIS Type 2 and Type 3 titanium and having ahigh residual ductility after being worked.

[0008] The inventors engaged in comprehensive studies on therelationship between the composition and properties of titaniummaterials of various chemical compositions, discovered a relationship ofstrength and ductility with components such as the oxygen equivalentvalue, Fe concentration, and nitrogen concentration, and perfected thepresent invention by limiting the range of chemical composition enablingprovision of a titanium auto part having almost the same strength leveland phase configuration as the conventional JIS Type 2 and Type 3titanium material and having a higher residual ductility after workingthe product compared with the past.

[0009] The present invention is based on this technical idea and has asits gist the following:

[0010] (1) A titanium auto part characterized by containing, by wt %,Fe: 0.15 to 0.5%, nitrogen: 0.015 to 0.04%, oxygen, and the balance oftitanium and unavoidable impurities, wherein when the Fe content is[Fe], the nitrogen content is [N], and the oxygen content is [O], theoxygen equivalent value Q=[O]+2.77 [N]+0.1 [Fe] is Q: 0.11 to 0.28%.

[0011] (2) A titanium auto part as set forth in (1) characterized inthat the oxygen equivalent value Q is 0.11 to 0.20%.

[0012] (3) A titanium auto part as set forth in (1) or (2),characterized in that the auto part is one of a muffler, exhaust pipe,motorcycle frame, motorcycle rear fork, and motorcycle front fork innerpipe.

BRIEF DESCRIPTION OF DRAWINGS

[0013]FIG. 1 shows a muffler and exhaust pipe of a motorcycle using atest example of the present invention.

[0014]FIG. 2 shows a motorcycle frame of a motorcycle using a testexample of the present invention.

[0015]FIG. 3 shows a motorcycle rear fork of a motorcycle using a testexample of the present invention.

[0016]FIG. 4 shows a motorcycle front fork inner pipe of a motorcycleusing a test example of the present invention.

[0017]FIG. 5 shows the appearance of a motorcycle using a test exampleof the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0018] The inventors engaged in intensive studies on the relationshipbetween the composition and properties in titanium materials of variouscompositions and as a result discovered the following importantphenomenon. That is,

[0019] [1] If the amount of Fe added exceeds 0.5%, a large amount ofβ-phase (bcc crystal structure) different in crystal structure from theα-phase of the matrix is produced, and the modulus of elasticity andheat expansion characteristics become remarkably different from the caseof containing only the conventional α-phase as a main phase, but if theamount of Fe added is not more than 0.5%, the amount of production ofthe β-phase is small and there is no need for readjusting the shape ofthe part, the production process, etc. for securing compatibility withthe surrounding parts. That is, it is possible to produce auto parts bythe same designs and production processes as in the past.

[0020] [2] Even with an amount of Fe of a maximum of 0.5% with littleproduction of the β-phase, if the amount added is 0.15% or more,addition of up to 0.04% of nitrogen enables achievement of an increasein strength without causing a reduction in the ductility. Alternatively,it is possible to improve the ductility without causing a reduction inthe strength. That is, it is possible to improve the balance of strengthand ductility.

[0021] The present invention was achieved based on the above twodiscoveries. Note that in the above discovery [2], if the amount of Feadded exceeds 0.6%, the effect disappears once, but if over 0.9%, theamount of the β-phase superior in the balance of strength and ductilityincreases, so an improvement appears in the relationship of strength andductility due to the combined addition of Fe and nitrogen again. Thiseffect, however, is effective for the high strength alloy such asdescribed in Japanese National Publication (Tokuhyo) No. 8-833292(International Publication No. WO96/33292), but is excessively strongfor a titanium material of a strength of the JIS Type 2 or Type 3 classcovered by the present invention and conversely impairs the ductility orworkability sought in titanium materials of this class, so is preferablynot applied to a material of a level of strength covered by the presentinvention.

[0022] Further, as explained in the above [1], in this case, since theamount of the β-phase is large, a need ends up arising for readjustingthe part shape or production process to secure compatibility with thesurrounding parts. Sufficient working cannot necessarily be performedwith a conventional production process.

[0023] The reasons for limiting the contents of oxygen, nitrogen, and Fein the above (1) will be explained next.

[0024] In the above (1), the amount of Fe added was made 0.15 to 0.5%.At least 0.15% was added because to improve the ductility withoutreducing the strength by the combined addition with nitrogen or toimprove the strength without reducing the ductility, it is necessary toadd at least 0.15% of Fe as described in the above discovery [2].

[0025] If adding more than 0.5%, however, not only does the effectdisappear, but also, as explained in [1], a large amount of β-phase (bcccrystal structure) different in crystal structure from the α-phase (hcpcrystal structure) of the matrix is produced, the modulus of elasticityand the heat expansion characteristics become remarkably different fromthe case of containing only the conventional α-phase as the main phase,so a need arises to readjust the part shape or production process etc.to secure compatibility with the surrounding parts, and it becomesdifficult to produce an auto part by a design or production process asin the past.

[0026] Further, in the above (1), the amount of nitrogen added was made0.015 to 0.04%. The reason is as follows. That is, if the amount of thenitrogen added is less than 0.015%, almost no improvement is recognizedin the relationship between the strength and ductility due to thecombined addition of Fe and nitrogen, and it is not possible to increasethe ductility without reducing the strength or increase the strengthwithout impairing the ductility. Therefore, in the present invention(1), the amount of nitrogen added was made at least 0.015%. Further, ifthe amount of the nitrogen added exceeds 0.04%, compounds of Ti andnitrogen are produced—resulting in a remarkable drop in ductility andthe effect of improving the relationship between strength and ductilityis erased. Therefore, the upper limit on the amount of nitrogen addedwas made 0.04%.

[0027] Further, in the above (1), the amount of oxygen added was made avalue giving an oxygen equivalent value Q=[O]+2.77 [N]+0.1 [Fe] of 0.11to 0.28.

[0028] Here, [Fe], [N], and [O] show the content of Fe, the content ofnitrogen, and the content of oxygen. The “oxygen equivalent value” is anindicator comprehensively showing the ability of oxygen, nitrogen, andFe to increase the strength of titanium. When making the ability of aunit wt % of oxygen to strengthen titanium as “1”, it is shown that aunit wt % of nitrogen exhibits a strengthening ability of 2.77 timesthat or that a unit wt % of Fe exhibits one of 0.1 time that.

[0029] In the present invention, Q was made a range of 0.11 to 0.28because making the value of Q this range makes it possible to achieve alevel of strength equal to that of the current JIS Type 2 and Type 3covered by the present invention. That is, when Q is less than 0.11, thestrength is too low and it is not possible to obtain a material having astrength of the general JIS Type 2 class actually being marketed, it isnecessary to reduce the relative concentration of oxygen in thetitanium, and it is necessary to use an expensive low oxygen sponge.Further, if Q is over 0.28, the strength becomes too high and theproblem arises that cold working becomes more difficult compared withthe JIS Type 3 class currently on the market.

[0030] A titanium material comprised of the chemical composition limitedas described above has a strength equal to that of the conventional JISType 2 to JIS Type 3 and has a ductility higher than these. Auto partscomprised of this titanium material has a higher residual ductilityafter cold working compared with even an auto part comprised of theconventional JIS Type 2 or Type 3 and can be used with greaterconfidence. Further, since the phase configuration enables part designssimilar to those of the conventional JIS Type 2 and Type 3, there is noneed to readjust the part shape etc. to secure compatibility with thesurrounding parts and production by designs and production processeslike in the past becomes possible.

[0031] Next, in the above (2), the range of the value of the oxygenequivalent value Q was made a range of 0.11 to 0.20. A greater ductilityis required in the production process in a titanium auto part of therange of the present invention (1) in the case of a soft material oftensubjected to more severe cold working. That is, if applying the presentinvention to a titanium auto part having a Q value in the range of 0.11to 0.20 of the soft level of strength corresponding to JIS Type 2, theeffect is particularly strongly manifested.

[0032] Note that the titanium spoken of in the present invention issubstantially comprised of Ti other than the oxygen, nitrogen, Fe, andunavoidable impurities. Here, the “unavoidable impurities” indicates theless than 0.05% of Ni or Cr, the less than 0.015% of carbon, the lessthan 100 ppm of hydrogen, etc. entering from the stock sponge titaniumor scrap titanium used or from the surface in the middle of theproduction process.

[0033] Further, in the above (1) and (2), when the amount of Fe added isa maximum of 0.3%, the compositions of the present invention are thosebelonging to JIS Type 2 or Type 3. As explained in the section of thePrior Art, commercially pure titanium of JIS Type 2 and Type 3 actuallycirculating on the market has a high Fe content of about 0.1% or so anda nitrogen content of a high 0.015% or so. While falling with the sameJIS standard, the material is different.

[0034] Further, when the Fe content exceeds 0.3%, the result is a lowalloy titanium material not falling under the category of JIS Type 2 orType 3.

[0035] Next, the above (3) limits the auto part to a muffler, exhaustpipe, motorcycle frame, motorcycle rear fork, or motorcycle front forkinner pipe. These parts have the appearances shown in FIGS. 1 to 4. Forexample, taking as an example a motorcycle, the material is used for thelocations shown in FIG. 5 (muffler (A), exhaust pipe (B), frame (C),rear fork (D), or front fork inner pipe (E)). These parts perform thefunctions shown in Table 1. By making these parts out of titanium, thebody is made lighter in weight and the fuel economy and operability areimproved. Further, as shown in Table 1, these parts are formed by coldworking such as pressing (restriction, expansion, punch stretching),bending, pipe expansion, etc. They are auto parts where particularlygood effects of the present invention can be expected.

EMBODIMENTS

[0036] Below, the present invention will be explained in detail byembodiments based on test examples.

[0037] (Test 1)

[0038] Titanium pipes (1 mm thickness, 48 mm diameter) of thecompositions shown in Test Nos. 1 to 11 of Table 2 etc. are used toproduce mufflers and evaluate the production characteristics by exactlythe same method as JIS Type 2. Test strips of 10 mm width and 150 mmlength are cut out in the longitudinal direction, and tensile tests areconducted at a gauge length of 50 mm to measure the tensile strength andelongation. The locations where the test pieces were taken are shown inFIG. 1. The test results are shown in Table 2.

[0039] In Table 2, Test Nos. 5, 8, 9, and 11 of examples of the presentinvention all could be sufficiently worked into mufflers and had thesame extent of strength and at least 2% higher residual elongationcompared with conventional commercially pure titanium (comparativeexample) having the same oxygen equivalent values, having no Fe ornitrogen deliberately added, and containing only unavoidable amounts ofFe and nitrogen present in stock sponge titanium.

[0040] That is, Test No. 5 give at least 2% higher residual elongationcompared with Test No. 4 while Test Nos. 8, 9, and 11 give the samecompared with Test No. 6, that is, the effect of the present inventionis exhibited. Further, Test Nos. 1 and 3 give extremely high residualelongations of at least 18%. In particular, Test No. 3 has an equaltensile strength and at least 2% higher residual elongation comparedwith conventional commercially pure titanium having the same oxygenequivalent value, having no Fe or nitrogen deliberately added, andcontaining only unavoidable amounts of Fe and nitrogen present in stocksponge titanium. However, both Test Nos. 1 and 3 only give insufficientstrength levels far below the tensile strength of the most general useJIS Type 2 material of Test No. 4. The sponge titanium used also ishigher in production costs due to use of a high purity materialcontaining only a maximum of 0.05 wt % of oxygen. The effect of thepresent invention cannot be sufficiently exhibited. This is because theoxygen equivalent value Q was lower than the 0.11 of the lower limit inthe present invention.

[0041] Test Nos. 7 and 10 only give tensile strengths and residualelongations equal to those of conventional commercially pure titanium(Test No. 6) having the same oxygen equivalent value, having no Fe ornitrogen deliberately added, and containing only unavoidable amounts ofFe and nitrogen present in stock sponge titanium. The effect of thepresent invention is not sufficiently exhibited. This is because theamount of Fe added in the case of Test No. 7 or the amount of nitrogenadded in the case of Test No. 10 was less than the lower limit of thepresent invention.

[0042] (Test 2)

[0043] Titanium pipes (2.0 mm thickness, 32 mm diameter) of thecompositions shown in Test Nos. 12 to 22 of Table 3 etc. are used toproduce motorcycle frames and evaluate the production characteristics byexactly the same method as JIS Type 2 or Type 3. Test strips of 10 mmwidth and 150 mm length are cut out in the longitudinal direction fromthe locations shown in FIG. 2, the ends of the test pieces are processedto gripping parts by bending, and tensile tests are conducted at a gaugelength of 50 mm to measure the tensile strength and elongation. The testresults are shown together in Table 3. Note that at the initial stage ofthe tensile test, the curved test pieces gradually deformed to becomestraight from the curved states and then were elongated and broke.

[0044] In Table 3, Test Nos. 13, 14, 17, 19, and 20 of examples of thepresent invention all could be sufficiently worked into mufflers and hadat least 2% higher residual elongation values comprised withconventional commercially pure titanium (comparative example) having thesame oxygen equivalent values and strength levels, having no Fe ornitrogen deliberately added, and containing only unavoidable amounts ofFe and nitrogen present in stock sponge titanium.

[0045] That is, Test Nos. 13 and 14 give the same extent of strength andat least 2% higher elongation compared with Test No. 12 (conventionalJIS Type 2), Test No. 17 gives the same compared with Test No. 16(conventional JIS Type 3), and Test Nos. 19 and 20 give the samecompared with Test No. 18 (conventional JIS Type 3). That is, the effectof the present invention is exhibited.

[0046] On the other hand, Test No. 15 ends up cracking during work. Thereason was as follows: That is, the amount of Fe added exceeded theupper limit of the present invention, so a large amount of β-phase (bcccrystal structure) different in crystal structure from the α-phase (hcpcrystal structure) of the matrix was produced, and the modulus ofelasticity and heat expansion characteristic became remarkably differentfrom conventional pure titanium containing only the α-phase as the mainphase. However, since the parts were produced by the same design andproduction process as those of conventional pure titanium withoutadjusting the shape of the part or adjusting the shaping method forsecuring compatibility with the surrounding parts, cracking ended upoccurring during working.

[0047] Further, Test No. 21 also ended cracking during working. This isbecause the amount of nitrogen added exceeded the upper limit of thepresent invention and therefore compounds of Ti and nitrogen wereproduced, the ductility was impaired, and the effect of the presentinvention could not be achieved.

[0048] Test No. 22 also ended up cracking during working, but this wasdue to the Q ending up exceeding 0.28 and therefore the strengthbecoming too high, the ductility relatively falling, and cold workingbecoming difficult compared with the JIS Type 3 class circulating on theusual market.

[0049] (Test 3)

[0050] Titanium pipes (1 mm thickness, 38 mm diameter) of thecompositions shown in Test Nos. 23 to 24 of Table 4 etc. are used toproduce exhaust pipes and evaluate the production characteristics byexactly the same method as JIS Type 2. Curved test strips of 10 mm widthand 150 mm length are cut out in the longitudinal direction from thelocations shown in FIG. 1, the ends of the test pieces are processed togripping parts by bending, and tensile tests are conducted at a gaugelength of 50 mm to measure the tensile strength and elongation. The testresults are shown together in Table 4. Note that at the initial stage ofthe tensile test, the curved test pieces gradually deformed to becomestraight from the curved states and then were elongated and broke.

[0051] In Table 4, Test No. 24 of an example of the present inventioncould be worked without causing cracking etc. by exactly the same methodas the usual pure titanium JIS Type 2 (Test No. 23), had a level ofstrength equal to that of ordinary pure titanium (Test No. 23), but hada residual elongation exceeding it.

[0052] (Test 4)

[0053] Titanium pipes (1.5 mm thickness, 32 mm diameter) of thecompositions shown in Test Nos. 25 to 26 of Table 4 etc. are used toproduce motorcycle rear forks and evaluate the productioncharacteristics by exactly the same method as JIS Type 3. Test strips of10 mm width and 150 mm length are cut out in the longitudinal directionfrom the locations shown in FIG. 3, the ends of the test pieces areprocessed to gripping parts by bending, and tensile tests are conductedat a gauge length of 50 mm to measure the tensile strength andelongation. The test results are shown together in Table 4. Note that atthe initial stage of the tensile test, the curved test pieces graduallydeformed to become straight from the curved states and then wereelongated and broke.

[0054] In Table 4, Test No. 26 of an example of the present inventioncould be worked without causing cracking etc. by exactly the same methodas the usual pure titanium JIS Type 3 (Test No. 25), had a level ofstrength equal to that of ordinary pure titanium (Test No. 25), but hada residual elongation exceeding it.

[0055] (Test 5)

[0056] Titanium pipes (3 mm thickness, 32 mm diameter) of thecompositions shown in Test Nos. 27 to 28 of Table 4 etc. are used toproduce motorcycle front fork inner pipes and evaluate the productioncharacteristics by exactly the same method as JIS Type 3. Test strips of10 mm width and 150 mm length are cut out in the longitudinal directionfrom the locations shown in FIG. 4, and tensile tests are conducted at agauge length of 50 mm to measure the tensile strength and elongation.The test results are shown together in Table 4.

[0057] In Table 4, Test No. 28 of an example of the present inventioncould be worked without causing cracking etc. by exactly the same methodas the usual pure titanium JIS Type 3 (Test No. 27), had a level ofstrength equal to that of ordinary pure titanium (Test No. 27), but hada residual elongation exceeding it. TABLE 1 Notation in FIG. 5 Fig. no.and part name Role Material Working 1 A, muffler Silencing of enginePipes/plates/ Pressing (drawing, exhaust noise bars/wiresbore-expanding/ stretch-expanding) Welding 1 B, exhaust pipe Pipeconnecting from Pipe bending engine to muffler Welding 2 C, frame Frameof body of Pipe/plates/ Pressing (drawing, motorcycle bars/wiresstretch-expanding) 3 D, rear fork Rear wheel suspension Welding 4 E,front fork Part of front wheel Pipes Pipe expansion inner pipesuspension parts

[0058] TABLE 2 Chemical composition (wt %) Oxygen Tensile Testequivalent strength Elongation no. Oxygen Nitrogen Fe Q* **Workability(MPa) (%) Remarks 1 0.0225 0.015 0.16 0.08 Good 372 19.2 Comp. ex. 20.0812 0.005 0.05 0.10 Good 392 16.5 Comp. ex. 3 0.0425 0.015 0.16 0.10Good 395 18.6 Comp. ex 4 0.1012 0.005 0.05 0.12 Good 490 11.5 Comp. ex 50.0529 0.017 0.20 0.12 Good 492 13.6 Inventions 2, 3 6 0.1412 0.005 0.050.16 Good 513 10.2 Comp. ex. 7 0.0778 0.025 0.13 0.16 Good 514 10.5Comp. ex 8 0.0738 0.025 0.17 0.16 Good 515 13.7 Inventions 2, 3 9 0.07960.020 0.25 0.16 Good 516 13.7 Inventions 2, 3 10 0.0990 0.013 0.25 0.16Good 516 10.4 Comp. ex. 11 0.0879 0.017 0.25 0.16 Good 517 13.8Inventions 2, 3

[0059] TABLE 3 Chemical composition (wt %) Oxygen Test equivalent L-Elongation no. Oxygen Nitrogen Fe Q* **Workability direction (%) Remarks12 0.1612 0.005 0.05 0.18 Good 538  9.6 Comp. ex. 13 0.0858 0.025 0.250.18 Good 540 11.9 Inventions 2, 3 14 0.0851 0.018 0.45 0.18 Good 54111.8 Inventions 2, 3 15 0.0701 0.018 0.60 0.18 Poor — — Comp. ex 160.2112 0.005 0.05 0.23 Good 560  8.5 Comp. ex 17 0.1308 0.025 0.30 0.23Good 562 10.7 Inventions 1, 3 18 0.2328 0.008 0.05 0.26 Good 591  8.0Comp. ex 19 0.1558 0.025 0.35 0.26 Good 590 11.2 Inventions 1, 3 200.1225 0.037 0.35 0.26 Good 592 11.5 Inventions 1, 3 21 0.1059 0.0430.35 0.26 Poor — — Comp. ex. 22 0.1869 0.030 0.30 0.30 Poor — — Comp. ex

[0060] TABLE 4 Chemical composition (wt %) Oxygen Test equivalent L-Elongation no. Part name Oxygen Nitrogen Fe Q* **Workability direction(%) Remarks 23 Exhaust pipe 0.1412 0.005 0.05 0.16 Good 515 8.3 Comp.ex. 24 Exhaust pipe 0.0796 0.020 0.25 0.16 Good 518 10.6 Inventions 1, 325 Rear fork 0.2412 0.005 0.05 0.26 Good 587 15.7 Comp. ex. 26 Rear fork0.1558 0.025 0.35 0.26 Good 585 19.1 Inventions 1, 3 27 Front fork0.2412 0.005 0.05 0.26 Good 592 17.8 Comp. ex inner pipe 28 Front fork0.1558 0.025 0.35 0.26 Good 593 21.0 Inventions inner pipe 1, 3

INDUSTRIAL APPLICABILITY

[0061] As explained above, by applying the present invention, it ispossible to provide a titanium auto part comprised of a titaniummaterial having a strength equal to the conventional JIS Type 2 and Type3 and of a phase structure enabling production by the same parts andprocesses as a conventional JIS Type 2 and Type 3 titanium material andpossible to obtain extremely advantageous effects industrially speaking.

1. A titanium auto part characterized by containing, by wt %, Fe: 0.15to 0.5%, nitrogen: 0.015 to 0.04%, oxygen, and the balance of titaniumand unavoidable impurities, wherein, when the Fe content is [Fe], thenitrogen content is [N], and the oxygen content is [O], the oxygenequivalent value Q=[O]+2.77 [N]+0.1 [Fe] is Q: 0.11 to 0.28%.
 2. Atitanium auto part as set forth in claim 1 characterized in that theoxygen equivalent value Q is 0.11 to 0.20%.
 3. A titanium auto part asset forth in claim 1 or 2, characterized in that the auto part is one ofa muffler, exhaust pipe, motorcycle frame, motorcycle rear fork, andmotorcycle front fork inner pipe.